A Tire Pressure Monitoring System (TPMS) is used for detecting pressure of a tire and maintaining the pressure at an appropriate level. The TPMS includes a sensor and a control device. The sensor installed in a tire detects pressure of the tire and transmits, to the control device installed in a vehicle, a sensing value and a unique ID of the sensor by using an RF module. The control device receives, from the sensor, RF data and vehicle information to determine a state of each tire and a state of a receiver and to transmit, to a display device recognizable by a driver, information to be displayed.
The TPMS may be classified into a high end system and a low end system according to a display method for a driver. The high end system displays pressure of each tire or displays a location of a tire of which pressure is low to a driver. The low end system just displays a warning message to a driver when even one of tires of a vehicle has low pressure.
To implement the high end system, information on a location of a sensor installed in each tire should be transmitted from the sensor, and an installation location of the sensor should be determined by using information utilizable by a control device.
U.S. Pat. No. 7,010,968 (Schrader) discloses one of TPMSs according to the related art. According to this system, Z/X-axis accelerometers are installed in left and right tires, and by using a principle in which Z/X-axis accelerometer signals of the left and right positions have different phases, the left and right positions are distinguished from each other. Further, by using a principle in which strength of an RF signal of a sensor, which is detected by a control device, is inversely proportional to the square of a distance to the sensor, the front and rear positions are distinguished from each other.
According to this system, a dual-axis accelerometer for distinguishing the left and right positions is needed, thereby requiring more space and increasing cost. Further, the speed of a vehicle and flatness of a road are limited in order to detect a phase difference between signals. Moreover, the control device for distinguishing the front and rear positions is required to be installed in such a location that the strength of the RF signal of the sensor is not reduced due to a chassis or electronic device of a vehicle or a driver. Therefore, this device is installed in a limited location such as a bumper, which requires additional wires and a waterproof function, causing an increase in cost and a limitation in space.
To overcome these limitations, it has been proposed that a function for detecting operation information (rotation speed) of a tire is added to a sensor in order to determine a location by comparing the operation information with wheel speed sensor information used for a chassis controller (ABS/ESC). According to this proposal, a Z-axis accelerometer is installed in the sensor in order to detect a wheel speed on the basis of a period of a signal of the accelerometer by using a phenomenon in which the signal of the accelerometer is outputted as a sine wave according to rotation of a wheel. Then, data on the wheel speed are transmitted to a control device by using an RF module, and the control device compares each wheel speed detected by a wheel speed sensor used for the chassis controller (ABS/ESC) with the received wheel speed so as to determine the location.
However, according to this proposal, as shown from the strength of the signal of the Z-axis accelerometer, a wheel speed offset occurs in the Z-axis accelerometer according to the rotation speed of a wheel. Therefore, in the case of high-speed rotation, due to a relatively great offset in comparison with the sine wave of the signal, it is difficult to detect the period of the sine wave. Further, an additional circuit for detecting the period of the sine wave is needed.